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United States Patent |
5,352,342
|
Riffe
|
October 4, 1994
|
Method and apparatus for preventing corrosion of metal structures
Abstract
A system for preventing corrosion of a surface of a metal structure in
contact with a corrosive environment comprising:
(a) a conductive zinc silicate coating in conductive contact with at least
part of the surface, wherein the conductive zinc silicate coating forms an
interfacial layer between the surface and the corrosive environment; and
(b) means for imparting a net negative bias to the metal structure, wherein
the means comprise a power supply means having a negative terminal
directly coupled to the metal structure and a positive terminal coupled to
a portion of the metal structure, remote from the negative terminal, by
way of a capacitor or resistor,
and a method of preventing corrosion using the system.
Inventors:
|
Riffe; William J. (510 Fisher St., Morehead City, NC 28557)
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Assignee:
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Riffe; William J. (Morehead City, NC);
Graham; Otho L. (Morehead City, NC)
|
Appl. No.:
|
034783 |
Filed:
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March 19, 1993 |
Current U.S. Class: |
205/735; 204/196.26; 204/196.27; 204/196.36; 205/740 |
Intern'l Class: |
C23F 013/00 |
Field of Search: |
204/147,196
|
References Cited
U.S. Patent Documents
5009757 | Apr., 1991 | Riffe et al. | 204/196.
|
5055165 | Oct., 1991 | Riffe et al. | 204/196.
|
Other References
PCT Search Report No. PCT/US94/02801, dated Jun. 2, 1994.
|
Primary Examiner: Niebling; John
Assistant Examiner: Mayekar; Kishor
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Claims
What is claimed as New and Desired to be Secured By Letters Patent of the
United States is:
1. A system for preventing corrosion of a surface of a metal structure in
contact with a corrosive environment, said system comprising:
(a) a conductive zinc silicate coating in conductive contact with at least
part of said surface, wherein said conductive zinc silicate coating forms
an interfacial layer between said surface and said corrosive environment;
and
(b) means for imparting a net negative electrostatic bias to said metal
structure, said means comprising a power supply means having a negative
terminal directly coupled to said metal structure and a positive terminal
coupled to said metal structure, at a position remote from said negative
terminal, by way of a capacitor;
wherein said surface with said conductive zinc silicate coating has an
active electronic barrier in a metal-oxide-p-semiconductory configuration
which inhibits a net transfer of electrons from said surface to oxidizing
species.
2. The system of claim 1, wherein said metal structure comprises a metal
selected from the group consisting of ferrous metals and conductive
non-ferrous metals.
3. The system of claim 2, wherein said metal is steel.
4. The system of claim 2, wherein said metal is aluminum.
5. The system of claim 1, wherein said power supply means delivers from 0.5
to 30 V to said system.
6. The system of claim 1, wherein said power supply means delivers from 10
to 20 V to said system.
7. The system of claim 1, wherein said power supply means is a direct
current power supply means selected from the group consisting of batteries
and solar cells.
8. The system of claim 1, wherein said power supply means is an alternating
current power supply means.
9. The system of claim 7, wherein said power supply means is a battery.
10. The system of claim 7, wherein said power supply means is a solar cell.
11. The system of claim 1, wherein said metal structure is selected from
the group consisting of metal vehicle parts, bridge members, railroad
coupling mechanisms, refineries, containers and metal towers.
12. The system of claim 1, wherein said conductive zinc silicate coating
comprises zinc in an amount of from 80-92% by weight based on dry coating.
13. The system of claim 1, wherein said conductive zinc silicate coating
comprises zinc in an amount of from 85-89% by weight based on dry coating.
14. A method for preventing corrosion of a surface of a metal structure in
contact with a corrosive environment, said method comprising:
inducing and maintaining a net negative electrostatic bias on said metal
structure, wherein said surface of said metal structure has a conductive
zinc silicate coating such that said zinc silicate coating is in
conductive contact with at least part of said surface and forms an
interfacial layer between said surface and said corrosive environment,
wherein said net negative electrostatic bias is sufficient to prevent
corrosion of said surface having said conductive zinc silicate coating
thereon by providing an active electronic barrier in a
metal-oxide-p-semiconductor configuration which inhibits a net transfer of
electrons from said surface to oxidizing species.
15. The method of claim 14, wherein said net negative bias is induced and
maintained by a means comprising a power supply means having a negative
terminal directly coupled to said metal structure and a positive terminal
coupled to a remote portion of said metal structure by way of a capacitor.
16. The method of claim 15, wherein said power supply means delivers from
0.5 to 30 V to said metal structure.
17. The method of claim 15, wherein said power supply means delivers from
10 to 20 V to said metal structure.
18. The method of claim 15, wherein said power supply means is a direct
current power supply means selected from the group consisting of batteries
and solar cells.
19. The method of claim 15, wherein said power supply means is an
alternating current power supply means.
20. The method of claim 18, wherein said power supply means is a battery.
21. The method of claim 18, wherein said power supply means is a solar
cell.
22. The system of claim 11, wherein said containers are selected from the
group consisting of storage silos and storage bins.
23. The system of claim 11, wherein said metal vehicle parts are metal
parts of a vehicle selected from the group consisting of cars, trucks,
tanks, marine vehicles, trains and airplanes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods and systems for
preventing corrosion of metal structures.
2. Discussion of the Background Art
In the construction of large metal structures, steel remains the economic
choice of materials. Unfortunately, steel has a tendency to corrode over
time.
A variety of methods for controlling corrosion have evolved over the past
several centuries, with particular emphasis on methods to extend the life
of metallic structures in corrosive environments. These methods typically
include protective coatings which are used principally to upgrade the
corrosion resistance of ferrous metals, such as steel, and some nonferrous
metals, such as aluminum, and to avoid the necessity for using more costly
alloys. Thus, they both improve performance and reduce costs. However,
such protective coatings typically have several pitfalls.
Protective coatings fall into two main categories. The largest of these
categories is the topical coating such as a paint, that acts as a physical
barrier against the environment. The second category consists of
sacrificial coatings, such as zinc or cadmium, that are designed to
preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a
primary purpose of mitigating and preventing corrosion. Each process is
different: cathodic protection prevents corrosion by introducing an
electrical current from external sources to counteract the normal
electrical chemical corrosion reactions whereas coatings form a barrier to
prevent the flow of corrosion current or electrons between the naturally
occurring anodes and cathodes or within galvanic couples. Each of these
processes provided limited success. Coatings by far represent the most
widespread method of general corrosion prevention (see Leon et al U.S.
Pat. No. 3,562,124 and Hayashi et al U.S. Pat. No. 4,219,358). Cathodic
protection, however, has been used to protect hundreds of thousands of
miles of pipe and acres of steel surfaces subject to buried or immersion
conditions.
The technique of cathodic protection is used to reduce the corrosion of the
metal surface by providing it with enough cathodic current to makes its
anodic disillusion rate become negligible (for examples, see Pryor, U.S.
Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No.
4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No.
4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). The cathodic
protection concept operates by extinguishing the potential difference
between the local anodic and cathodic surfaces through the application of
sufficient current to polarize the cathodes to the potential of the
anodes. In other words, the effect of applying cathodic currents is to
reduce the area that continues to act as an anode, rather than reduce the
rate of corrosion of such remaining anodes. Complete protection is
achieved when all of the anodes have been extinguished. From an
electrochemical standpoint, this indicates that sufficient electrons have
been supplied to the metal to be protected, so that any tendency for the
metal to ionize or go into solution has been neutralized.
However, there is a strong divergence of opinion between the proponents of
paint coatings and the proponents of cathodic protection. Proponents of
"coatings only" are often on one side discounting the advantages of
cathodic protection, claiming that a good, well applied coating is the
only necessary protection for steel. On the other side, the proponents of
cathodic protection often claim that any immersed or buried metal
structure can best be protected by the installation of a well engineered
cathodic protection system. There are many conditions under which one type
of protection may be superior to the other. However, under most of the
more commonly occurring conditions, the best conventional corrosion
protection is actually a combination of both concepts. But even when the
two concepts are combined, problems still occur.
Inorganic zinc coatings have functioned previously by allowing a limited
sacrificial corrosion of the incorporated zinc to provide sufficient free
electrons to preclude the removal of electrons from the underlying steel
during the corrosion process. Under normal conditions of exposure in an
industrial atmosphere, in the United States, a two mil coating could be
expected to protect steel from corrosion for from four to six years
depending upon the weather. Submerged in a salt water environment, the
same coating would provide from one to two years of corrosion prevention
to the underlying steel. When used to protect girder type highway bridges
or automobile underbodies, inorganic zinc has proven less successful
because the continuous contact with chloride ions and moisture accelerates
the sacrifice of the metallic zinc in the coating and blisters off the
various organic top coats.
The destruction of organic top coats over the inorganic zinc coatings has
been particularly severe in those cases where impressed cathodic
protection was attempted simultaneously. In general, the problem of top
coating with organic top coats over inorganic zinc coatings has been the
eventual intrusion of water through the organic coating that contacted the
zinc and released sufficient hydrogen from the corrosion process to
blister off the organic top coat. The destruction of organic top coats
over inorganic zinc coatings has been particularly severe in those cases
where impressed cathodic protection was attempted simultaneously. When
impressed cathodic protection was applied to the system, the electric
potential caused electroendomesis and blistered off the top coat even more
quickly than when no current was applied.
In galvanic corrosion, those metals that have conducting or n-type
semiconducting products (passive films, scales, and so forth) are at risk
from the standpoint of localized attack caused by the ability of the
surface films to support cathodic reactions and hence to provide a
galvanic influence to the corrosion process. That is not to say that
materials with nonconductant or p-type semiconducting films are not at
risk. Aluminum is an obvious exception, as are results with very thin
films (nickel and copper) that support electron transfer by tunnelling or
surface states. It can be said, however, that the galvanic influence to
localized corrosion, when it occurs in aqueous systems, requires a cathode
material capable of supporting reduction of H.sup.+. This is most likely
to be the case for n-type semiconductors, intrinsic or degenerate
conductors or for very thin films.
The products of corrosion, especially with solids, fall under three
different categories, based on their ability to serve as electrodes, these
three categories being insulators, semiconductors, and conductors. The
dividing line between categories is quite hazy and a particular oxide or
sulfide may exhibit a range of conductivity depending on its degree of
stoichiometry.
It has been previously shown that corrosion is generally the development of
a galvanic couple between anodic (oxidizing) sites and cathodic (reducing)
sites upon a metallic surface immersed in a conductive solution of
ionizable compounds, such as seawater. This galvanic couple allows the
transfer of electrons through the corroding metal from the ions formed by
oxidation at the anodic sites to reducible ions at the cathodic sites. The
overall result is that metal is converted to its various compounds at the
anodes and reduction of various ions takes place at the cathodes, until
all of the original metal is converted to a lower chemical energy state.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a method for
preventing corrosion of metal structures by utilizing semiconductor
technology and with no external anode, no electrolyte, and no current
flow.
A further object of the present invention is to provide a system for
protecting metal structures from corrosion, wherein the system provides
long term protection with minimal system maintenance required.
A still further object of the present invention is to provide a system and
method for preventing corrosion of metal structures which are either
submerged in water, exposed to air, or a combination of both.
Another object of the present invention is to provide a system and method
for preventing corrosion of marine structures which also functions to
prevent fouling of the structures by marine organisms.
These and other objects have been satisfied by the discovery of a system
for preventing corrosion of a surface or surfaces of a metal structure in
contact with a corrosive environment, comprising:
(a) a conductive zinc silicate coating in conductive contact with at least
part of the exterior surface of the metal structure, wherein the
conductive zinc silicate coating forms an interfacial layer between the
exterior surface and the corrosive environment; and
(b) means for imparting a net negative bias to the metal structure, wherein
the means comprise a power supply means having a negative terminal
directly coupled to the metal structure and a positive terminal coupled to
a portion of the metal structure, remote from the negative terminal, by
way of a capacitor or resistor;
and the discovery of a corrosion prevention method comprising:
1) cleaning the external surface of a metal structure;
2) coating the external surface with an inorganic zinc silicate based
coating; and
3) inducing and maintaining a net negative bias on the metal structure.
BRIEF DESCRIPTION OF THE FIGURES
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same become better
understood by reference to the following detailed description when
considered in connection with the accompanying Figures, wherein:
FIG. 1 shows a schematic representation of the coating of the present
invention on an iron structure.
FIG. 2 shows a schematic representation of the porous microstructure of the
coating of the present invention.
FIGS. 3-8 show the apparatus configurations used to obtain the data
presented in Table II below.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a system for preventing corrosion of a
surface or surfaces of a metal structure in contact with a corrosive
environment, said structure having an exterior surface, said system
comprising:
(a) a conductive zinc silicate coating in conductive contact with at least
part of said exterior surface, wherein said conductive zinc silicate
coating forms an interfacial layer between said exterior surface and said
corrosive environment; and
(b) means for imparting a net negative bias to said metal structure, said
means comprising a power supply means having a negative terminal directly
coupled to said metal structure and a positive terminal coupled to a
portion of said metal structure, remote from the negative terminal, by way
of a capacitor or resistor.
The present invention further relates to a corrosion prevention method
comprising:
1) cleaning the external surface of a metal structure;
2) coating the external surface with an inorganic zinc silicate based
coating; and
3) inducing and maintaining a negative bias on the metal structure.
The present system comprises two interdependent components: (1) the zinc
silicate-based coating, and (2) a means for imparting a net negative bias
to the metal structure to which the coating is applied. In general the
zinc silicate-based coating is applied to the metallic surface after it
has been cleaned, preferably by grit blasting to a commercial blast
finish. When a metal surface is cleaned by grit blasting or comparable
methods, the surface will have numerous grooves or indentations of from
0.1 mil up to several mil in depth. The zinc-based coating of the present
invention should be applied at a depth of at least 2 mil greater than the
depth of the pits formed from the grit blasting of the metal, preferably
from 2 to 10 mil thickness, most preferably 7 to 9 mil thick.
The zinc silicate-based coating of the present invention can be the same
coating as disclosed in U.S. Pat. No. 5,009,757 to W. Rifle which is
hereby incorporated by reference. The basic building blocks of the
inorganic zinc coating are silica, oxygen, and zinc. In liquid form, they
are relatively small molecules of metallic silicate such as sodium
silicate or organic silicate such as ethyl silicate. These essentially
monomeric materials are crosslinked into a silica-oxygen-zinc structure
which is the basic film former or binder for all of the inorganic zinc
coatings. Suitable inorganic zinc coatings for use in the present
invention are the various commercially available alkyl silicate or alkali
hydrolyzed silicate types. One such commercially available paint is
Carbozinc D7 WB.TM. manufactured by Carboline, Inc.
There are essentially three stages in the formation of the inorganic zinc
coating. The first reaction is the concentration of the silicates in the
coating by evaporation, after the coating has been applied to the surface.
As the solvent evaporates, the silicate molecules and the zinc oxide come
in close contact and are in a position to react with one another. This
initial solvent evaporation provides for the primary deposition of film on
the surface of the metal structure. The evaporation may be performed by
any suitable means, such as applying heat, forcing air over the surface,
or natural evaporation.
The second reaction is the oxidation of the zinc and iron metal which
initiates the reaction of the zinc and iron oxides with the silicate
molecule to form a zinc silicate polymer.
The third reaction is the completion of the film reaction by the continuing
formation of zinc ions which react to increase the size of the zinc
silicate polymer and crosslink it into a very insoluble, resistant,
three-dimensional structure. This reaction continues indefinitely
throughout the life of the coating, reaching a useable level of
crosslinking within one to three days. Such a structure is shown
schematically in FIG. 1. Such a formation of a coherent thin film is a
relatively unique reaction in inorganic chemistry, since inorganic
materials generally do not form coherent thin films. The only other common
inorganic film is one formed by fusing inorganic material to a basic metal
in order to create a ceramic enamel.
Prior to application the inorganic zinc silicate coating should preferably
contain no more than 75% zinc dust or powder. Upon drying the film, the
zinc content of the dry film should be at least 80% by weight, preferably
80-92%, more preferably 85-89% by weight. If the zinc content of the dry
film is greater than 92% the integrity of the film is detrimentally
effected.
The development of semiconductor properties in zinc oxide appears to be
achieved through modification of the crystal lattice. The lattice of pure
zinc oxide consists of a periodic arrangement of zinc and oxygen ions
(ZnO). The charges of these ions constitute the strong ionic bond of the
crystal structure and are not available for conduction. With no free
electrons, the electrical conductivity is low and the material is an
insulator. One method of developing semiconductor properties in zinc oxide
is by inclusion of interstitial zinc atoms in zinc oxide which has been
partially reduced by reaction with reducing agents, such as carbon
monoxide or hydrogen at elevated temperatures (approximately
400.degree.-900.degree. C.). Each atom of oxygen on removal releases an
atom of zinc and two electrons. The zinc atom moves to the void space
between oxygen atoms, thus the designation "interstitial atom". The charge
on that atom and the disposition of the electrons have been the subject of
controversy among investigators for nearly half a century. It appears that
the interstitial zinc atom may carry variable amounts of charge (Zn,
Zn.sup.+, and Zn.sup.++), depending mainly on temperature, the number of
free electrons varies accordingly. At room temperature, for example, the
atom may be present as Zn.sup.+, leaving one free electron to serve as
charge carrier. The interstitial zinc ion Zn.sup.+ (termed "excess zinc")
contributes to the electrical conductivity of the crystal; in fact, some
investigators equate such conductivity with the concentration of excess
zinc.
It should be noted that the interfacial electric field generated in the
system of the present invention is not due to any externally applied
voltage, rather it is built-in at the metal-semiconductor. Metallic
surfaces host positive dipole layers when they are interfaced with
appropriately doped semiconductors to form metal and metal semiconductor
and metal-oxide (insulator)--semiconductor structures. These interfacial
space charge layers result in a built-in electric field and cause bending
of the electronic energy bands. The net band bending is defined as the
active electronic barrier. It may be added that by properly selecting the
semiconductor coating material for a metal surface, one can realize both
the traditional passive as well as the novel active barriers.
A metallic surface can also develop a positively charged dipole layer and
the associated active electronic barrier in a metal-oxide (insulator)-p
semiconductor (MOS) configuration. The active electronic barrier inhibits
the net transfer of electrons from the metal surface to the oxidizing
species, resulting in a lower probability of oxidation/corrosion.
Additionally, the electronic barrier may help in regions having micropores
and pinholes in the semiconductor layer. In these regions we expect a
finite electric field (due to field fringing effects) to retard the
transfer of electrons.
Zinc metal covered with zinc oxide appears to behave like a diode in that
electrons travel more easily from the base metal to the oxide than they do
from the oxide to the base metal. In the case of the inorganic zinc
silicate coating of the present invention, any electrons traveling from a
steel substrate must traverse an iron oxide layer, transfer to an iron
silicate layer, and pass through a zinc silicate layer to enter the zinc
metal. However, surface corrosion at the solid-liquid interface can occur
if zinc ions enter the liquid from zinc metal at the surface. To do so,
electrons must depart the zinc oxide at the anodic foci and travel to
cathodic areas through the zinc oxide/zinc silicate layers.
To inhibit corrosion of the metallic zinc at the coating surface, the
conventional approach would be to provide a surplus of electrons at the
zinc surface by supplying an external anode which is either galvanically
sacrificial or which provides an electrically impressed potential and
current flow counter to that of the corroding metal.
The method of the present invention provides an alternative means of
corrosion control by preventing the flow of electrons from ionizing zinc
to the surface/water interface. This is done by preventing the flow of
electrons from corroding anodic sites to cathodic sites and establishing a
minute current flow internal to the inorganic zinc silicate coating. In
the corrosion process the initial electrons and zinc ions are generated
from the interstitial zinc common to zinc oxide. However, the continued
replacement of these materials must come from the zinc metal through the
n-type semiconductor zinc oxide. In the connection between the zinc oxide
upon the zinc dust and the connection through the zinc silicate/sodium
silicate/iron silicate/iron oxide to the substrate iron in the steel there
is a n-p-n semiconductor activity.
When a pn junction diode is biased negative (-) on the p-side and positive
(+) on the n-side, current flow will be inhibited. For conventional
silicon diodes, the current flow drops below 1.0 .mu.A. This
simultaneously establishes a capacitance in the depleted region of the
junction of about 40 pF. Very similar values are obtained in the present
system. This is possible because if the substrate metal is biased negative
in a capacitive circuit, the pn junction of the inorganic zinc silicate
coating will see a positive charge from the initial movement of
interstitial electrons to the surface/water interface. Thus, the p-side
sees a negative potential and the n-side sees a positive potential and
corrosion current ceases except for a very slight amount of "back
current". This may be thought of as a semi-self-biasing mechanism. By
blocking the flow of electrons to the coating surface the sacrificial
corrosion of zinc, typical of inorganic zinc silicate coatings, is
inhibited and the life of the coating is greatly extended with no
reduction in the corrosion protection afforded the metal substrate.
The system of the present invention differs substantially from previous
corrosion control methods in that it eliminates the chemical
oxidation/reduction reactions, neither through simple electron replacement
as in conventional cathodic protection systems, nor by exclusion of the
chemical reactants as in conventional paint coatings, but by electronic
suppression of the interface current flow essential to corrosion. This
constitutes a total departure from previous practice in that there is no
paint film undergoing continuous atmospheric degradation to the point that
it not longer provides protective isolation for the underlying structure.
The zinc dust of the coating of the present invention forms a pn junction
where the zinc metal and zinc oxide interface, with the zinc oxide
becoming an n-type semiconductor and the zinc metal becoming a p-type
semiconductor. This effectively forms a field effect transistor (FET).
The completed coating is schematically shown in FIG. 2. FIG. 2 shows the
porous nature of the zinc silicate coating (4) of the present invention.
The zinc particles (1) are covered by a zinc oxide layer (2) with the
various oxide coated particles surrounded by an insoluble heavy metal
silicate binder (3). At the interface (5) between the coating and the
structure metal, is an insoluble metal silicate layer, which in the case
of a steel structure would be an insoluble iron silicate layer.
The structure of the zinc silicate coating of the present invention
resembles a metal oxide semiconductor field effect transistor (MOSFET).
All metal-oxide-semiconductor field effect transistors (MOSFET) are "n"
type or "p" type materials. A MOSFET has no electrical contact between the
source and the drain. A glass-like insulating layer separates the gate's
metal contact from the rest of the structure. It operates as follows: in a
"p-n" junction there is a space charge barrier. This space charge region
is one in which the normal carrier density is depleted by the
thermodynamic requirements for equilibrium at the junction. If the space
charge region includes a large fraction of the sample, the means of
modulating the resistance of that sample are readily available since by
varying the voltage on the junction the width of the space charge region
can be varied over a rather wide limit. By increasing the proper voltage
upon the gate one can decrease the cross-section of the conducting region
through which current may flow from source to drain. Because it is
difficult to extend the space charge barrier over large distances (greater
than 0.01 cm) such devices must be small or divided into such small
regions that the space charge barrier can extend over the entire
conducting region.
The potential distribution of various applied voltages on the gate follows
Poisson's equation:
d.sup.2 V/dx.sup.2 =4.pi..rho.(.chi.)/.kappa.
where p is the space charge density at the point x, V is the value of the
potential, V.sub.o the "pinch-off" voltage, and .kappa. is the inverse of
the ion atmosphere radius, 1/.kappa., where the ion atmosphere radius is
defined as the distance from the charged surface into the solution within
which the major portion of electrical interactions with the surface are
considered to occur. The ion atmosphere radius is also known as the Debye
length and is the effective thickness of the electrical double layer. The
system of the present invention obtains the required small regions by
virtue of the size of the zinc dust particles which are from 0.0007 to
0.0014 cm in diameter with about 0.0001 cm gate thickness.
The coating of the present invention is a p-metal adjacent to an n-oxide,
surrounded and insulated by a silicate. The entire structure acts as if it
were a steel substrate "gate" from which a field may be applied. Under
conventional sacrificial protection methods, the zinc contained in the
coating eventually depletes from the matrix resulting in the final failure
of the coating and the substrate steel begins to corrode. The depletion of
the zinc is caused by the loss of electrons through the zinc oxide layer
and the loss of zinc ions.
There is ample evidence for the semiconductor nature of the zinc silicate
coating of the present invention. Usually, when one increases the
temperature of metals, the resistance increases. However, when the zinc
silicate coating of the present invention is heated, its resistance
decreases, much like a typical semiconductor in which the heat affects the
movement of interstitial holes and electrons and thus increases current
flow.
Additionally, when the zinc silicate is substituted for the capacitor in a
R-C circuit arranged in series, the coating exhibits the characteristics
of a differentiator, indicating its transistor pn-junction capacitive
effect.
In a conventional FET, as the frequency of an impressed signal increases,
the current flow through the gate of the FET increases, due to the
inability of the relaxation time of the capacitor in a RC-circuit to have
sufficient time to cause the well known "pinch effect" to occur at the
gate of the FET. In the present system, as the frequency of an impressed
signal through the coating of the present invention is increased, the
resistance decreases, thus increasing current flow.
However, in a traditional FET, if the gate of the FET senses an external
field, the dimensions of the high impedance zone in the transistor of the
FET increase and the flow of holes or electrons across that boundary is
severely limited or even ceases entirely. Similar effects can be seen with
the coating of the present invention. Upon coating a steel plate with the
zinc silicate coating of the present invention, and connecting the
uncharged coated plate to an electrometer, containing an FET circuit, by
way of a contact lead, the electrometer indicates a flow of electricity
through the internal FET circuit of the electrometer. Upon impressing a
static charge on the coated metal plate, with an electrostatically charged
wand, the meter immediately indicates no current flow, since the electrons
provided by the charged wand are impressing on the internal FET circuit of
the electrometer and causing the gate of the FET to "pinch" off the
current flow. However, if the coated steel structure is biased by an
external source such as a battery, the zinc oxide/zinc particles of the
zinc silicate coating are seen to function, under bias, as if they were a
large number of tiny FETs and thereby block the flow of electrons from the
wand to the electrometer through the coated steel plate thus allowing the
internal FET circuit of the electrometer to show current flow once more.
When a conventional sacrificial system comes into contact with either water
or moisture in the air, the oxide layer on the zinc is penetrated and
anodic corrosion cells begin. The electrons left behind by the zinc ions
migrate through the oxide layer to surface reducing sites. In order to
halt the corrosion/ionization at the zinc/zinc oxide interface the travel
of electrons must be reduced. This is accomplished in the present
invention by the impression of a net negative bias across the system. When
this biasing field is applied to the substrate steel, the oxide layer is
closed to electron flow and consequently no ions can be produced and
corrosion ceases. Thus, the bias upon the substrate causes the coating to
act as a barrier to electron flow and reduces the corrosion of the zinc
particles by several orders of magnitude over conventional coating
systems.
Silicates are natural corrosion inhibitors in their own right. However, if
one relies merely on the presence of silicates to prevent corrosion in the
substantial absence of zinc and zinc oxide, such a coating would last for
only a matter of days due to the high solubility of the silicate. However,
one of the advantages gained by the presence of silicates in the
formulation of the present invention takes advantage of this natural
corrosion inhibition property, primarily in the event of a power loss to
the means for imparting a negative bias to the metal structure. In such a
case, the coating of the present invention would still provide protection
until power is restored, with the protection being enhanced by the
presence of the natural corrosion inhibition ability of the silicate.
The metal structure of the present invention can be any metal structure in
need of protection from corrosion. Examples of such metal structures
include metal vehicle parts, bridges, railroad coupling mechanisms,
containers, pipes and metal towers. Examples of metal vehicle parts
include metal parts of vehicles such as automobiles, airplanes, trains,
military land vehicles such as tanks, and ships and other marine vehicles.
As examples of containers are refinery containers, storage silos and
storage bins.
The amount of current traveling from the steel through the inorganic zinc
silicate system is minute. Some idea of the magnitude can be gained from
an analogy with the transfer of current from a steel pipe conductor to
adjacent water.
A current flowing in pipes containing water usually causes no accelerated
corrosion to the inside of the pipe. The high electrical conductivity of
CN compared to water (or seawater) makes it nearly impossible to generate
corrosion currents across the pipe/water interface which are sufficient to
accelerate corrosion. For example: The resistance of any conductor per
unit length equals p/A where p is the resistivity and A is the
cross-sectional area. Thus the ratio of current carried by a metal pipe
compared to that carried by the water it contains is equal to p.sub.w
A.sub.m /p.sub.m A.sub.w where subscripts W and m refer to water and
metal, respectively. For iron, pm is about 10.sup.-5 .OMEGA./cm and for
potable water pw may be 10.sup.4 .OMEGA./cm. Assuming that the
cross-sectional area of water is 10 times that of the steel pipe, it is
seen that if the current flowing through the pipe is 1A, only about
10.sup.-8 A is flowing through the water. This small current leaving the
pipe and entering the water causes negligible corrosion. If seawater is
transported instead with pw=20.OMEGA./cm, the ratio of current carried by
water to current carried by pipe is 2.times.10.sup.-5, indicating that
even in this case most of the current is carried by the metallic pipe and
there is very little stray current corrosion on the inner surface of the
pipe.
It is to be noted that the resistivity in fresh water is 10.sup.4 ohms and
for salt water only 20 ohms/cm and yet the transfer of current from the
very conductive steel to the water is 10.sup.-5 proportionately. In the
case of the structure of the inorganic zinc silicate coating, the
resistance is much greater as is shown in the table below.
TABLE I
______________________________________
Zinc Dust Concentration Versus Resistivity
Zinc Dust % by Weight
Resistivity in 100 ohms/sq in
______________________________________
95 12.7
90 4.0
85 2.5
80 10.5
75 290
70 1900
65 11,000
Clear 150,000
______________________________________
Values using the system of the present invention, when measured to ground
from the system of the present invention when submerged were 0.01 .mu.A
(10.sup.-8 A) or almost the equivalent values for the transfer of current
from a conductive pipe to adjacent flowing water.
The coating of the present invention can prevent corrosion in four distinct
ways: three conventional methods, and a fourth mechanism of
electrochemical origin which is the thrust of the present invention. In
the first conventional method, the zinc silicate coating acts as a typical
barrier coating preventing moisture from reaching the coated substrate.
Secondly when there are voids adjacent to the substrate and moisture does
penetrate through these voids free silica will function as an inhibitor as
follows:
Zinc can contribute to silica incorporation under situations approaching
those where it is useful as a corrosion inhibitor, and alkalinity is not
controlled (pH greater than or equal to 8). Under these conditions, the
following chemical reactions are thought to occur:
Na.sub.2 SiO.sub.3 +2H.sub.2 CO.sub.3 =2NaHCO.sub.3 +H.sub.2 SiO.sub.3
5Zn.sup.-2 +2HCO.sub.3 --+8OH.sup.- -2ZnCO.sub.3 .multidot.3Zn(OH).sub.2
+2H.sub.2 O
2ZnCO.sub.3 .multidot.3Zn(OH).sub.2 +3H.sub.2 SiO.sub.3 =2ZnCO.sub.3
.multidot.3ZnSiO.sub.3 +6H.sub.2 O
Thus, under alkaline conditions, the permeated fluid is basic at the
coating/zinc interface and there is no interruption of the chemical
reaction.
In the third conventional method, the zinc within the coating acts as a
sacrificial metal to provide cathodic protection as previously described.
The most probable mechanism of cathodic protection of steel and seawater
is a sufficient number of electrons from a preferred external source to
accommodate a cathodic reaction, such as oxygen reduction or hydrogen
evolution, over the whole surface of the metal being protected. In the
absence of cathodic protection, the electrons reacting with the oxygen at
the cathodic surfaces must be supplied by corrosion at the anodic areas
(metal substrate surface). As additional electrons are supplied from an
external source, the oxygen reduction reaction is accommodated by these
additional electrons and fewer are required from the original anodes. This
causes some of the original anodes to be converted to cathodes, and thus
the current reaching the cathodic surfaces from the remaining anodes
decreases as the external current increases, so that the total cathodic
current density does not change substantially until all of the anodes are
extinguished and the current density increases on the whole of the metal
surfaces.
In the three conventional methods of corrosion prevention discussed above
the idea is to first prevent a galvanic couple by refusing the moisture
access to the metallic surface, by using highly waterproof paint films.
Secondly, inhibitors may be used to interrupt the chemical reactions at
corrosion sites, and thirdly, some other metal may be sacrificed
oxidatively to protect the more desirable metal of the substrate from
corrosion.
However, the method of the present invention relies on a fourth mechanism
of electrochemical origin that has never been exploited to control
corrosion. The present method relies on the interruption of the flow of
electrons from the anodic ionizing sites through the metal to the reducing
cathodic sites. Thus, if it is impossible for a metal atom in the metallic
matrix to lose an electron, then ionization will cease when a certain
level of negative charge is established within the metal. In conventional
impressed cathodic protection systems, there is a deluge of electrons
introduced from some external source so that all of the metallic surface
becomes cathodic. In so doing, there is an ongoing reduction of positive
ions in the adjacent solution such that gasses are evolved and various
precipitates leave the solution. The biggest drawback to such cathodic
protection is that an abundant and continuous supply of electricity is
required.
In the system of the present invention, an electrically negative bias is
established within the inorganic zinc silicate coating upon the metal
substrate, by applying the charge to the substrate. Because the coating
matrix is conductive, a charge field is induced within the zinc metal
component of the zinc powder. The zinc/zinc oxide forms a weaken junction,
wherein the applied charge and slight surface ionization causes a reverse
bias, with the result that transfer of electrons from the zinc/zinc oxide
to reduction sites is effectively blocked. A negative charge is thus
developed in the Zn metal of the coating, with the coating having a
partial positive charge overall compared to the base metal, and corrosion
ceases. This differs substantially from cathodic protection, in that
electrons are denied access to the coating/water interface, rather than
being provided in excess, and the applied electrical charge is static, as
opposed to having a current flow.
One significant advantage obtained in the present invention is that by
inhibiting the corrosion of the zinc within the matrix of the inorganic
zinc silicate coating, the life of the coating will be extended to be many
times longer than that of conventional zinc silicate coating protection
systems. While this would be possible to achieve under water through the
application of cathodic current, it would require substantial current and
would be very difficult to control. Further, such a cathodic system would
give no benefit to protecting structures above the water where there is
essentially no galvanic couple. The method of the present invention
functions internally to the coating and thus prevents atmospheric
corrosion where the corroding medium is nothing more than moisture in the
air that is insufficient to enable cathodic protection. This becomes
extremely important in protecting such surfaces as the internal surfaces
of modern ships, where designs to provide increased strengths have
concomitantly increased corrosion prone areas, and in protecting
automobile parts, bridges, airplanes, and trains.
For example, the method of the present invention may be used to protect the
internal surfaces of modern ships where the condensation is most corrosive
due to its high saline content and where, at the same time, there is
insufficient moisture for cathodic protection systems to function. Without
the impressed negative bias of the present invention, the zinc in the
inorganic zinc silicate coating would quickly leach out and be eroded away
by the flow of condensate to the bilges. However, upon the application of
an electrically negative bias to the metallic substrate, this leaching is
effectively halted.
Additionally, the charge upon the substrate steel of the ship, provides no
greater interference to shipboard electronics than turning on a light bulb
within the ship, nor would it yield a detectable signal to hostile
detection devices, since the field does not radiate perceptibly beyond the
coating. The absorbance characteristics of zinc are well known and are
often used for EM shielding and electronics enclosures. Thus, there would
be no measurable EM radiation from shore-based structures to which the
present system is applied.
In the present system, unlike impressed cathodic protection, there is
virtually no current flow. Instead, the metal substrate is charged
electrostatically in the same manner as in capacitance charging and the
metal substrate and zinc oxides are reverse biased in the manner of pn or
npn semiconductors so that current flow virtually ceases. Because the
electrical field is internal to the coating and structure, there is little
osmotic attraction of hydroxyls into the organic top coats which may be
applied, nor is there any accretion of calcareous build-ups upon submerged
surfaces as with impressed cathodic protection systems.
The means for imparting a net negative bias in the present system can be
any means capable of providing a net negative bias sufficient to cause the
net electron flow to favor electron flow into the substrate metal, rather
than out of the substrate metal. Suitable means for imparting the net
negative bias include direct current (DC) power supply means such as
batteries, preferably 12 Volt batteries, and solar cells and alternating
current (AC) power supply means. The power supply means used preferably
delivers a voltage of from 0.5 to 30 V, most preferably 10 to 20 V. The
power supply means of the present invention has a negative terminal
directly coupled to the metal structure to be protected. The positive
terminal of the power supply means is coupled to the metal structure by
way of a capacitor or resistor, to a portion of the metal structure remote
from the negative terminal connection. Since the present invention does
not rely on creation of current flow, which drops off as the distance
between terminals increases, the distance between the terminals is not
critical, so long as the positive and negative terminals do not touch one
another and short out the power supply. The positive terminal connection
is preferably made to a location on the metal structure from 0.01 meter to
30 meters from the location of the negative terminal connection, most
preferably from 5 to 10 meters from the location of the negative terminal
connection.
The source for the net negative bias can be either a direct current or
alternating current, depending upon the desired application. In
applications for metal structures which are not in contact with bodies of
water, it is preferred to use a negative bias at all times in order to
prevent corrosion and prolong the life of the coating. However, for
underwater uses, it is advantageous to use an alternating bias in which a
negative bias is applied for 70 to 100% of the cycle, preferably greater
than 85% of the cycle with 0 to 30%, preferably less than 15% of the cycle
using a positive bias. Use of an alternating bias in this manner provides
the anti-corrosion benefits of the present invention along with the
anti-fouling characteristics of the coating of the present invention
disclosed in U.S. Pat. No. 5,009,757.
The method of the present invention is self-tending for the life of the
system. There are no currents or potentials to monitor and control
periodically as there would be in a conventional cathodic protection
system. Further, there is no possibility that the present system can go
out of control and severely damage the supporting structures as can occur
in an impressed cathodic protection system. The only effective reduction
in the life of the coating would therefore come from wind and water-borne
abrasion. Since the abrasion resistance of the coating is somewhat better
than that of galvanize, the life expectancy of the coating can be extended
to the range of several decades.
Having generally described this invention, a further understanding can be
obtained by reference to certain specific examples which are provided
herein for purposes of illustration only and are not intended to be
limiting unless otherwise specified.
EXAMPLES
In order to demonstrate the effect of negatively biasing both grounded and
ungrounded assemblies according to the present invention, the following
experiments were conducted.
Plates of 3/16" mild steel which has been sandblasted and coated with
0.004" of the inorganic zinc silicate coating of the present invention
were attached to electrodes and placed in tanks filled with water and
ordinary table salt added to the same concentration as sea water. The
plate to be negatively biased, in accordance with the present invention,
was connected to a 12 Volt DC battery with a 1.0 .mu.F capacitor between
the plate and the positive terminal of the battery, as shown in FIGS. 3-8.
FIGS. 3-8 show the configurations used to obtain the data presented in
Table II. In each case a ground plate, prepared in the same manner as the
biased plate, is used, with the ground plate connected through a long
copper wire to ground. FIG. 3 shows the configuration used to place a
grounded negative bias on the sample plate using an impressed cathodic
charge. FIG. 4 shows the configuration used to provide a grounded negative
bias on the sample plate using an impressed anodic charge. FIGS. 5 and 6
show the configurations used to impose an ungrounded negative bias on the
sample plate using impressed cathodic and anodic charges, respectively.
FIGS. 7 and 8 show the configurations for imposing an ungrounded positive
bias on the sample plate using impressed cathodic and anodic charges,
respectively. The results obtained are presented in Table II. The dry cell
used to impress the anodic or cathodic potentials on the biased assemblies
produced 1.6 V DC. 0.8 V DC is the commonly accepted value for ionization
suppression of steel/zinc in salt water. The accompanying .mu.A values
reported were approximated from that value in comparison to the 1.603 V DC
dry cell battery.
TABLE II
__________________________________________________________________________
GROUNDED ASSEMBLY
UNGROUNDED ASSEMBLY
Potential
Negative Bias
Negative Bias
Positive Bias
Cat/An
Measured
Theoretical
Measured
Theoretical
Measured
Theoretical
__________________________________________________________________________
Cathodic
1.603 Vdc
0.8 Vdc
1.603 Vdc
0.8 Vdc
1.603 Vdc
0.8 Vdc
42.5 .mu.A
101 .mu.A
40.48 .mu.A
117 .mu.A
40.50 .mu.A
81.8 .mu.A
7.92 K.OMEGA.
10.68 K.OMEGA.
9.78 K.OMEGA.
Anodic
1.603 Vdc
0.8 Vdc
1.603 Vdc
0.8 Vdc
1.603 Vdc
0.8 Vdc
39.00 .mu.A
70 .mu.A
39.10 .mu.A
86 .mu.A
42.80 .mu.A
104 .mu.A
11.3 K.OMEGA.
9.30 K.OMEGA.
7.65 K.OMEGA.
__________________________________________________________________________
It would be expected that the imposition of an anodic potential to the
biased assembly would produce a tendency for the zinc and the coating to
ionize into the adjacent water. From the corrosion preventive point of
view, it would be desirable to minimize this type of electrochemical
activity. However, as shown in the above table, the system of the present
invention which provides the greatest resistance to ionization (highest
K.OMEGA.) under anodic pressure is the grounded assembly having a negative
bias, followed in descending order by the ungrounded assembly having a
negative bias and the ungrounded assembly having a positive bias. In
ungrounded assemblies, those having a negative bias show better resistance
to oxidation and reduction than do the assemblies with a positive bias.
In making the above measurements on the ungrounded assemblies, the positive
bias measurements were made first. Following measurements of the positive
bias numbers, the bias was reversed to be a negative bias. Less than 10
minutes after reversal, measurements for the negative bias assemblies were
made. This indicates that there is no lengthy time delay required for the
effects of biasing to be established after application of the bias.
In comparing a bias applied only to the surface of the metal in isolation
to a bias applied through the means of a ground, the grounded system was
found to be approximately 12.7 K.OMEGA. more resistive to electron flow
than the ungrounded system.
In an additional experiment, tin metal plates were substituted for both the
biased and unbiased plates of the previous experiments at the same time.
Both biased and unbiased tin metal plates exhibited the same resistance to
an impressed current, indicating that the observed effects are due to the
zinc silicate coating of the present system rather than the circuitry used
in the experiments.
Obviously, numerous modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the invention may
be practiced otherwise than as specifically described herein.
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